Recovery of caprolactam from nylon 6 scrap, wherein the scrap is substantially free of non-nylon 6 materials, has been practiced for at least twenty years. In general, nylon 6 is depolymerized by heating at elevated temperatures, usually in the presence of a catalyst and/or steam. The caprolactam produced is removed as a vapor stream. An extensive review of the field has been given by L. A. Dmitrieva et al, Fibre Chemistry, Vol. 17, No. 4, March 1986, pp 229-241. Depolymerization of hydrolyzable polymers that are produced as scrap during the manufacture of fiber, chip, film or molded articles is also described in U.S. Pat. No. 4,605,762 to Mandoki. The process includes introducing the polymeric scrap into a hydrolyzer at a temperature of 200.degree. to 300.degree. C. and a pressure of at least 15 atmospheres, wherein high pressure steam is introduced into the lower portion of the hydrolyzer below the polymeric scrap. An aqueous solution of the products of the hydrolysis reaction is withdrawn from an upper portion of the hydrolyzer.
However, for multi-component mixtures or composites that contain nylon 6 as weld as other components, recovery of caprolactam is complicated by the presence of the other components. These other components and/or their decomposition products generated under conventional nylon 6 depolymerization conditions interfere with the isolation of caprolactam of adequate purity, thus necessitating expensive additional purification steps.
As an example of a product including nylon 6 as well as substantial amounts of other materials, carpet products made of a nylon 6 face fiber also include a backing (support) material which may include jute, polypropylene, latex (such as a styrene-butadiene rubber (SBR)) and a variety of inorganic materials such as calcium carbonate, clay or hydrated alumina fillers. Typically, the face fiber constitutes only 20-50% by weight of the carpet, with the rest of it consisting of the backing materials. In addition, the fiber may contain dyes, soil repellants, stabilizers and other compounds added during fiber and/or carpet manufacture. Waste carpet may also contain a host of other impurities, which will collectively be referred to herein as "dirt".
These non-nylon 6 components interfere with caprolactam recovery. For example, one of the most difficult problems is that alkaline components, such as the calcium carbonate filler, neutralize the acidic catalysts, such as phosphoric acid, that are conventionally used to promote nylon 6 depolymerization, thus requiring the use of increased amounts of catalyst. A further problem is that for waste material that includes substantial amounts of non-nylon 6 components, such as waste carpet material, the non-nylon 6 materials are difficult to process or handle.
It would be particularly beneficial if an inexpensive method could be developed for the recovery of caprolactam from multi-component composites or materials that include nylon 6, such as carpets, especially a method in which byproducts of non-nylon 6 components could be readily separated and handled.